1. Field of the Invention
The present invention relates to optically readable cards carrying a digital data message which is used to control access to a service, e.g. to a pay-TV program which may be broadcast over the air or else transmitted by electrical or optical fiber cable. The invention also relates to a system which uses such cards to control access to such a service.
2. Description of the Prior Art
Cards carrying an encoded message for providing the cardholder with access to a service are well known. Although the details vary from system to system, the same general principles apply, for example, to tickets that give access to public transport, to credit cards that give access to bank note dispensers, and to subscriber cards that enable unscrambler (or decoder) equipment to unscramble (decode) pay-TV programs.
The particular size of the card used may vary from system to system, as may the material from which the "card" is made. Thus, for example, credit cards are generally made of plastic and occupy rather more than twice the area of railway tickets which, in contrast, are generally made of cardboard. For the purpose of the present invention, credit cards and railway tickets are merely common examples of the kind of card to which the invention is applicable, and neither the size nor the material of the card is critical.
There are several ways of encoding data on such cards: the cards may be perforated with perforations that can be read optically or by electrical contact; the cards may include a magnetic medium which is read like a tape recording as the card moves past a read head; the cards may include a semiconductor memory chip; the cards may have optically readable marks; etc. In a system using such cards, the cards are read by suitable apparatus which transmits one or more digital words from the card for further use by the rest of the system. The transmission is generally serial, i.e. bit-by-bit.
In a simple system, the card acts very like a key in a lock, in other words data stored on the card is merely compared with data stored in the system (e.g. by loading a word in parallel from a read only memory (ROM) into a shift register and then comparing that word bit-by-bit with the word read from the card). If the two words are identical, then the card holder is given access to the service, otherwise access is denied.
In a more complicated system, data read from the card is stored in one or more suitable memories of the system, and in addition to simply determining whether access is to be given or denied, the system can select a particular service from a plurality of services, set a limit (e.g. a credit limit) on a service, or check whether the card-holder knows a "pass word".
All such systems are open to abuse by means of counterfeit cards. While it is relatively difficult for most of the people to counterfeit cards with magnetically encoded data (i.e. most people do not have access to equipment for reading and writing digital recordings), optically readable cards are much easier to counterfeit. This is true regardless of whether the optically readable card encodes data by means of punched holes or by means of visible marks (like the bar codes used for retail checkout points). A genuine card can readily be copied with sufficient accuracy to produce a working counterfeit by using hand tools or a photocopier.
Preferred embodiments of the present invention provide an optically readable card carrying a digital data message which is substantially impossible to counterfeit without special photographic or opto-electronic equipment.